Method for measuring rate of fluid absorption of plaster molds

ABSTRACT

A solid state device for measuring the water absorption ability of a plaster mold includes a water reservoir, a capillary member and a pair of thermistors, one of which is designed to measure temperature changes in response to water flow in the capillary member during the measuring process.

United States Patent Krostewitz [54] METHOD FOR MEASURING RATE OF FLUIDABSORPTION OF PLASTER MOLDS {72] Inventor: Wolfgang Krostewitz,Somerville, NJ.

[73] Assignec: American Standard Inc., New York, NY.

[22] Filed: May 28, 1970 21 Appl. No.: 41,435

52 U.S.Cl ..73 73,73 204 51 1nt.Cl. ...G0lf 1/00 58 Field ofSearch..73/204,7 3

[56] References Cited UNITED STATES PATENTS Adams ..73/204 51 May 30,1972 3,216,249 11/1965 Joel ..73/204 3,085,431 4/1963 Yerman et a1...73/204 2,947,938 8/ 1960 Bennett ..73/204 X Primary Examiner-Jerry W.Myracle Att0rneySheldon H. Parker, Tennes 1. Erstad and Robert G. Crooks571 ABSTRACT A 'solid state device for measuring the water absorptionability of a plaster mold includes a water reservoir, a capillary memberand a pair of thermistors, one of which is designed to measuretemperature changes in response to water flow in the capillary memberduring the measuring process.

10 Claims, 2 Drawing Figures Patented May 30, 1972 3,665,755

FIGI

INVENTOR. Wolfgang Krosfewirz QJZM7% AT TO RNEY METHOD FOR MEASURINGRATE OF FLUID ABSORPTION OF PLASTER MOLDS BACKGROUND OF THE INVENTION 1.Field of the Invention This invention relates to a device for measuringthe water absorption rate of plaster molds. More particularly, thepresent invention relates to a device for measuring the rate ofabsorption of water by plaster molds from a clay suspension in water.

2. Description of the Prior Art In the fabrication of large thin-walledobjects such as crucibles, retorts and sanitary ware as well as in themanufacture of fine earthenware, porcelain and the like, it has beencommon for many years to employ slipcasting. Briefly, this techniqueinvolves pouring a suspension of suitable composition and concentrationinto a preshaped plaster mold, the highly porous nature of which removeswater from the slip by capillary action. As water is continuouslyremoved from the slip, the slip becomes locally concentrated at theslip-mold interface, forming a solidified clay layer at the surface ofthe plaster mold. Growth of the solidified region is permitted tocontinue by this process until such time as the desired thickness isattained. This end may be reached either by stopping the process bydraining off excess liquid, or alternatively, by adding additional slipto compensate for the water removed until such time as the total areawithin the mold is filled with a solid concentrate. The former and morepopular process is termed drain casting and the latter solid casting.

. Although the slip casting process is, indeed, a simple techniquerequiring little skill, numerous difficulties are encountered in thecontrol fo the rate and uniformity of skin growth. Unfortunately, thesedimentation volume and the solid concentration of the skin are subjectto considerable variation, such being attributed in large measure to therate of removal of water from the suspension. Accordingly, workers inthe art have long sought in vain to devise a technique or apparatuscapable of predetermining the water absorption rate of a plaster mold,thereby providing a ready means for predicting the suitability of aparticular mold for the intended purpose.

SUMMARY OF THE INVENTION In accordance with the present invention, thesedesiderata are met by a novel device which is capable of measuring therate of absorption of water by a plaster mold. Briefly, the subjectdevice includes a container having disposed therein a measuringthermistor and a reference thermistor, a reservoir of water and acapillary member through which water flows during the measuring processfrom the water reservoir into the plaster mold. Appropriate circuitry isprovided to permit heating of the thermistors. When the device isoperated as described herein, the temperature changes in the measuringthermistor caused by the flow of water passing on its way to the moldare detected and, by means of suitable calibration techniques, arecorrelated to the rate of water being absorbed by the mold.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more readilyunderstood by reference to the following detailed description taken inconjunction with the accompanying drawings wherein:

FIG. 1 is a schematic representation of the device of the invention; and

FIG. 2 is a circuit diagram of the apparatus described.

DETAILED DESCRIPTION OF THE INVENTION With reference now moreparticularly to FIG. I, there is I l7 and an outlet 18 (maintainedoutside the container) upon which is affixed a hose member 19 which isdesigned to avoid leakage between the capillary and the mold of interest20. Measuring thermistor 13 is physically located in a region of flow,that is, an area wherein a representative rate of flow can be perceived.Accordingly, it is desirable to situate thermistor 13 in the capillary15, at inlet 17, or in the region immediately surrounding the inlet.Reference thermistor 14 is located in water reservoir 12 in a region ofgeneral quiescence, that is a region in which there is no perceptibleflow of water during the absorption process.

The device of interest is completed by means of the necessary circuitryand power supply shown in FIG. 2. FIG. 2 shows the circuit of interestwherein measuring thermistor 13 (T and reference thermistor 14 (T areeach positioned as one leg of a conventional Wheatstone bridge, R, and Rbeing the remaining resistors in the bridge. The bridge is supplied withpower by means of power source P preferably a direct current powersource. The circuit also includes a measuring means S for measuring thepartial difference over S which is proportional to the rate of flow ofwater through the capillary and into the mold.

The Wheatstone bridge utilized in the subject device, as noted above,include a pair of thermistors T and T and a pair of resistors R and RIdeally, it is desirable that the resistance value of T and T be of thesame magnitude. Similarly, in the interest of enhanced sensitivity, itis desirable that R, and R be of the same magnitude as the thermistors,the remaining resistor being a variable resistance which is capable ofbeing adjusted for zero signal output at zero water flow.

In the operation of the device, the power is initially turned on andunder the influence of the applied power the thermistors are heated upto an equilibrium temperature. During this period, the signal outputfrom the bridge is zero since both thermistors are heating at equalrates, that is, the heat exchange of both T and T is equal, slightdeviations having been cancelled by the variable resistor R Themeasuring process is then initiated by contacting hose member 19 with asuitable plaster of Paris mold, such as 20, whose rate of waterabsorption it is desired to measure. Upon contact, the absorptive forcesof the mold immediately begin to withdraw water from the capillary. Thewater maintained in the water reservoir 12 remains stagnant for allpractical purposes except in the so-called region of flow surroundinginlet 17 wherein water is admitted to the capillary to replace thatvolume of water removed by the mold. Accordingly, the heat exchange ofmeasuring thermistor 13, T will begin to change and, consequently, itstemperature will change. However, the temperature of referencethermistor 14 remains stable in the area of relative stagnation in waterreservoir 12. Thus, an imbalance is created in the Wheatstone bridge asindicated by a signal output from S (in FIG. 2).

Calibration of the device may be effected by creating a known flow ofwater in the capillary by any well-known procedure, as for example, byinjecting an air bubble into the capillary, and measuring the signaloutput, and the time it takes for the air bubble to traverse a givendistance.

The air bubble is used as a marker, so that a point in the water streamcan be identified. Alternatively, an immiscible dye could be injectedinto the water stream so as to permit the velocity of the water to bemeasured.

The capillary tube is calibrated in any convenient manner, as forexample, by the conventional technique of weighing the amount of waterbetween two markers in the capillary tube.

The flow rate is varied so that a required range of calibration ofsignal output vs. flow rate can be obtained. The flow rate can be variedby any convenient technique, such as, changing the height of the supplyreservoir of the water or by using a variable speed water supply pump.

Alternatively, calibration can be carried out by weighing the amount ofwater which flows through the capillary tube in a unit of time. Signaloutput vs. flow rate can thus be calibrated for a variety of flow rates.

It should also be understood that the term capillary member" is a termof convenience and is not intended to be given a narrow technicalinterpretation.

The tube diameter is essentially determined by the size (cross-sectionalarea) of the tip 22 of the hose member 19. The greater the surface areaprovided for the absorption of the fluid, by the porous body the greatercan be the diameter of the capillary tube. The linearity of the flowrate vs. the water absorption rate is improved by using larger diametercapillary tubes.

As an exemplary embodiment of the subject device follows:

A rectangular glass container, 1% X 2 x 2% inches having a capillarymember of one-fourth inch inside diameter (I.D.), (including an inletand an outlet) disposed therein and emerging from a side wall wasemployed. The container was filled with 0.117 liters of water. Next, ameasuring thermistor T was inserted in the water adjacent to the inletof the capillary and a reference thermistor T inserted in the water nearthe far end of the capillary. Both T and T manifested a resistance ofkilohms. Thereafter, T and T were connected as legs of a Wheatstonebridge having a pair of resistors R and R each of 10 kilohms resistanceand a power source P comprising a dry cell battery of 12 volts was alsoconnected to the bridge. Next, the apparatus so constructed wascontacted with a typical plaster of Paris mold by means of a hose memberaffixed to the outlet of the capillary and water flow initiated. (In thedevice described, calibration was effected prior to measurement). Basedupon the prior calibration, water flow for the mold described was foundto be approximately 100 microliters per second per 30 squaremillimeters.

It will be appreciated by those skilled in the art that the above devicemay be varied without departing from the spirit and scope of theinvention as, for example, by varying the value of the thermistors orresistors in the bridge, the power source, etc. Additionally, it may beconsidered desirable to employ an additional water reservoir tocontinuously replenish water removed from the capillary and main waterreservoir by the mold. Further, the fluid can be a gas as well as anyliquid such water. Thus, while the system has particular applicabilityand utility in connection with testing the condition of a plaster mold,it can also be applied to analyzing gas flow from porous bodies otherthan plaster molds.

It should also be noted that, the tube diameter is essentially relatedto the size of the tip 22. The greater the surface area provided forabsorption the larger can be the tube diameter. This is not critical,however, but rather affects linearity. Obviously, it is more expedientto employ a linear system, but in any event, a complete calibration canbe obtained.

For convenience the term capillary tube is employed, but it should beunderstood that the tube diameter can exceed the maximum diameter for acapillary tube, which for water is approximately one-sixteenth of aninch.

What is claimed is:

1. A method of measuring the water absorption rate of a plaster moldcomprising,

a. providing fluid communication between a fluid reservoir and a surfaceof said plaster mold;

b. measuring the fluid flow rate between the fluid reservoir and saidsurface of said plaster moldi c. wherein said fluid flow rate isdirectly related to the water absorption rate of said plaster.

2. The method of claim 1 wherein said step of measuring the fluid flowrate between the fluid reservoir and said'surface of said plastercomprising measuring the change in resistance of a measuring thermistorposition in said fluid.

3. A method in accordance with claim 2 wherein said measuring thermistoris situated proximate said mold surface.

4. A method inaccordance with claim 2 wherein said measuring thermistoris situated in said fluid reservoir.

5. A method in accordance with claim 2, further comprising, heating saidmeasuring thermistor and a reference thermistor.

6. The method of claim 1 wherein the aforesaid fluid communication takesthe form of a flow duct having a flow area substantially less than thatof the reservoir, whereby the linear flow rate in the duct issubstantially greater than the linear flow rate in the reservoir.

7. The method of claim 6 wherein measuring step (b) com priseselectrically comparing the electrical resistances offered by a firstthermistor disposed in the flow duct and a second thermistor disposed inthe reservoir.

8. A method of measuring the fluid-absorption capability of a porousplaster mold body comprising the steps of connecting the porous plasterbody to a fluid source having negligible pressure head, and measuringthe rate of fluid flow from the source into the body due to capillaryaction.

9. The method of claim 8 wherein the fluid source comprises a liquidreservoir elevated only a few inches above the connection to the fluidbody, whereby only a small pressure head is established in thereservoir.

10. The method of claim 9 wherein the fluid connection between thereservoir and the porous body has a flow area substantially less thanthat of the reservoir, whereby the linear flow rate in the fluidconnection is substantially greater than the linear flow rate in thereservoir; flow rate measuring step comprising the substep of comparingthe electrical resistances offered by a first thermistor disposed in thefluid connection and a second thermistor disposed in the reservoir.

1. A method of measuring the water absorption rate of a plaster moldcomprising, a. providing fluid communication between a fluid reservoirand a surface of said plaster mold; b. measuring the fluid flow ratebetween the fluid reservoir and said surface of said plaster mold; c.wherein said fluid flow rate is directly related to the water absorptionrate of said plaster.
 2. The method of claim 1 wherein said step ofmeasuring the fluid flow rate between the fluid reservoir and saidsurface of said plaster comprising measuring the change in resistance ofa measuring thermistor position in said fluid.
 3. A method in accordancewith claim 2 wherein said measuring thermistor is situated proximatesaid mold surface.
 4. A method in accordance with claim 2 wherein saidmeasuring thermistor is situated in said fluid reservoir.
 5. A method inaccordance with claim 2, further comprising, heating said measuringthermistor and a reference thermistor.
 6. The method of claim 1 whereinthe aforesaid fluid communication takes the form of a flow duct having aflow area substantially less than that of the reservoir, whereby thelinear flow rate in the duct is substantially greater than the linearflow rate in the reservoir.
 7. The method of claim 6 wherein measuringstep (b) comprises electrically comparing the electrical resistancesoffered by a first thermistor disposed in the flow duct and a secondthermistor disposed in the reservoir.
 8. A method of measuring thefluid-absorption capability of a porous plaster mold body comprising thesteps of connecting the porous plaster body to a fluid source havingnegligible pressure head, and measuring the rate of fluid flow from thesource into the body due to capillary action.
 9. The method of claim 8wherein the fluid source comprises a liquid reservoir elevated only afew inches above the connection to the fluid body, whereby only a smallpressure head is established in the reservoir.
 10. The method of claim 9wherein the fluid connection between the reservoir and the porous bodyhas a flow area substantially less than that of the reservoir, wherebythe linear flow rate in the fluid connection is substantially greaterthan the linear flow rate in the reservoir; flow rate measuring stepcomprising the substep of comparing the electrical resistances offeredby a first thermistor disposed in the fluid connection and a secondthermistor disposed in the reservoir.